Channichthyidae
| Icefish | |
|---|---|
 |
|
| Chaenocephalus aceratus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Perciformes |
| Suborder: | Notothenioidei |
| Family: |
Channichthyidae T. N. Gill, 1861 |
| Genera | |
The crocodile icefish or white-blooded fish (Channichthyidae) comprise a family of notothenioid fish found in the Southern Ocean around Antarctica and southern South America, where water temperatures remain relatively stable (generally ranging from -1.8 to +2.0 °C). Sixteen species of crocodile icefish are currently recognized.
All icefish are believed to be piscivorous but can also feed on krill. Icefish are ambush predators; thus, they can survive long periods between feeding and often consume fish up to 50% of their own body length. Maximum body lengths of 25–50 cm have been recorded in these species.
Icefish blood is colorless because it lacks hemoglobin, the oxygen-binding protein in blood. Channichthyidae are the only known vertebrates to lack hemoglobin as adults. Although they do not manufacture hemoglobin, remnants of hemoglobin genes can be found in their genome. The hemoglobin protein is made of two subunits (alpha and beta). In 15 of the 16 icefish species, the beta subunit gene has been completely deleted and the alpha subunit gene has been partially deleted. In only one of the icefish species, Neopagetopsis ionah, there is a more complete, but still nonfunctional hemoglobin gene.
Red blood cells (RBCs) are usually absent, and if present, are rare and defunct.Oxygen is dissolved in the plasma and transported throughout the body without the hemoglobin protein. The fish can live without hemoglobin because of their low metabolic rates and the high solubility of oxygen in water at the low temperatures of their environment (the solubility of a gas tends to increase as temperature decreases). However, the oxygen-carrying capacity of their blood is less than 10% that of their relatives with hemoglobin.
Myoglobin, the oxygen-binding protein used in muscles, is absent from all icefish skeletal muscles. In 10 species, myoglobin is found in the heart muscle, specifically ventricles. Loss of myoglobin gene expression in icefish heart ventricles has occurred at least four separate times.
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